'upstairs' originates from the combination of 'up' and 'stairs', where 'up' meant 'toward a higher place' and 'stairs' referred to 'a series of steps'.

'upstairs' changed from the Old English word 'upp' and 'stæger' and eventually became the modern English word 'upstairs'.

Initially, it meant 'toward a higher place via steps', and this meaning has largely remained the same in modern usage.